Update alternatives ошибка нет альтернатив для python

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The other day I decided that I wanted the command python to default to firing up python3 instead of python2.

So I did this:

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 3

$ sudo update-alternatives --config python

$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.5   3         auto mode
  1            /usr/bin/python2.7   2         manual mode
  2            /usr/bin/python3.5   3         manual mode

Press <enter> to keep the current choice[*], or type selection number: 0

And that all worked. Great! 🙂

$ python -V
Python 3.5.2

But it wasn’t long before I realised I had broken apt/aptitude when it comes to installing and removing python packages because apt was expecting python2 it transpired.

This is what happened.

$ sudo apt remove  python-samba
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n] 
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
  File "/usr/bin/pyclean", line 63
    except (IOError, OSError), e:
                             ^
SyntaxError: invalid syntax
dpkg: error processing package python-samba (--remove):
 subprocess installed pre-removal script returned error exit status 1
Traceback (most recent call last):
  File "/usr/bin/pycompile", line 35, in <module>
    from debpython.version import SUPPORTED, debsorted, vrepr, 
  File "/usr/share/python/debpython/version.py", line 24, in <module>
    from ConfigParser import SafeConfigParser
ImportError: No module named 'ConfigParser'
dpkg: error while cleaning up:
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 python-samba
E: Sub-process /usr/bin/dpkg returned an error code (1)

Eventually I guessed it wanted python2 as the default, so I undid my changes as follows:

$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.5   3         auto mode
  1            /usr/bin/python2.7   2         manual mode
  2            /usr/bin/python3.5   3         manual mode

Press <enter> to keep the current choice[*], or type selection number: 1

$ python -V
Python 2.7.12

And then apt worked again

$ sudo apt remove  python-samba
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n] 
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...

So I have had to leave it as defaulting to python 2 but I develop in python 3 and so would like my system to default to python 3 for when I run python and idle.

Can anyone tell me how I can achieve this without breaking apt?

My system is a Raspberry Pi 3B running Ubuntu:

Linux mymachine 4.4.38-v7+ #938 SMP Thu Dec 15 15:22:21 GMT 2016 armv7l armv7l armv7l GNU/Linux

(It’s actually an arm v8)

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"

On our Kali Linux (or any other Linux distribution) we might have installed different versions of Python. For using Python version 2.x we generally use python2 command, same as for using Python 3.x versions we use python3 command.

Here assume that we have installed multiple versions of Python3 installed on our system, like we have installed Python3.7 and Python 3.9 both on our Linux system for any reason. So whenever we want to use Python 3.9 we need to type command python3.9 because python3 command using Python 3.7 version as default.

Our advanced Linux users may know this problem and the solution, but this is for beginners.

How to check installed Python versions on Linux?

This can be easily done with a simple command on our Terminal window. The command is following:

ls /usr/bin/python*

In the following screenshot we can see that we have Python2.7, Python3.7 and Python3.9 installed on our system.

Problem

But we can see that python3 command is choosing Python3.7 version as default. But some updated tools needs Python3.9 to run. We can run python3.9 command, but it is annoying we should run python3 to run Python3 latest version, we may modify our .bashrc/.zshrc file but that will not be the correct solution.

We need to set our update-alternatives for python3.

We can check for the alternatives of python3 by running following command:

sudo update-alternatives --config python3

But here we might get an error «update-alternatives: error: no alternatives for python3«.

It means, first we need to set alternatives for python3.

Solved

To set the alternatives for python3 we need to run some commands on our terminal.

First of all we need to run the following command:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1

This command will add Python 3.7 on option 1.

Then we need to run following command:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 2

This command will add Python 3.9 on option 2

We can see this on the following screenshot:

Now we can again run the configure command to check and set the alternatives:

sudo update-alternatives --config python3

In the following screenshot we can see that now we can save the configurations now.

Here we can set the default version for the python3. Here automatically 0 is chosen for Python 3.9 version, we can go for it, otherwise instead of choosing by numbers we can run following command to choose the default python3 version:

sudo update-alternatives  --set python3 /usr/bin/python3.9

Now we can check python3 default version by using following command:

python3 -V

We can see that now our Python 3.9 version is set as default for python3 command:

«update-alternatives: error: no alternatives for python3» is a very common problem for beginners so we thought to write an entire article for it we got too much request to solve this on our Telegram DM. When Python 4 will release some versions of Python 4, we can use the same as we did for Python 3.

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В операционной системе Linux, основанной на Debian (Debian, Ubuntu, Mint, Kali и т.д.), может быть установлено несколько версий python. Вы можете запустить команду ls чтобы посмотреть версии, установленные на вашей системе:

$ ls /usr/bin/python*
/usr/bin/python  /usr/bin/python2  /usr/bin/python2.7  /usr/bin/python3  /usr/bin/python3.8  /usr/bin/python3.8m  /usr/bin/python3m

Чтобы проверить, какая версия python используется по умолчанию, выполните:

$ python --version
Python 2.7.8

Чтобы изменить версию python для отдельного пользователя, просто создайте alias для домашней директории пользователя. Откройте файл ~/.bashrc и добавьте новый alias чтобы изменить путь к исполняемому файлу python по умолчанию:

alias python='/usr/bin/python3.8'

После внесения изменений перелогиньтесь или перечитайте файл .bashrc:

$ . ~/.bashrc

Проверьте версию python по умолчанию:

$ python --version
Python 3.8.5

Если вы используете не bash, а другую оболочку, внесите алиас в соответствующий .rc-файл.

Изменение версии python для всей системы

Чтобы изменить версию python для всей системы, можно использовать команду update-alternatives. Залогиньтесь как пользователь root (или используйте sudo c далее описанными командами) и выведите список доступных альтернатив python на вашей системе:

# update-alternatives --list python
update-alternatives: error: no alternatives for python

Сообщение выше означает, что команда update-alternatives не распознает альтернативы python. Нужно обновить таблицу альтернатив для команды python, включая все имеющиеся в системе версии, например, python2.7 и python3.8:

# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode
# update-alternatives --install /usr/bin/python python /usr/bin/python3.8 2
update-alternatives: using /usr/bin/python3.8 to provide /usr/bin/python (python) in auto mode

Ключ --install принимает несколько аргументов, из которых может сделать символическую ссылку. Последний аргумент обозначает приоритет и, если вручную не указано, альтернатива с большим числом будет использована по умолчанию. В данном случае, мы указали приоритет 2 для /usr/bin/python3.8 и, как результат, /usr/bin/python3.8 был автоматически установлен командой update-alternatives как основной python.

# python --version
Python 3.8.5

Далее, можно вывести список всех альтернатив python:

# update-alternatives --list python
/usr/bin/python2.7
/usr/bin/python3.8

Теперь, в любое время, можно переключаться между альтернативами введя ниже указанную команду и вводя номер строки с альтернативой:

# update-alternatives --config python

# python --version
Python 2.7.8

В случае, если из системы будет удалена одна из версий python, её можно будет удалить из листинга update-alternatives. Например, удалим версию python2.7:

# update-alternatives --remove python /usr/bin/python2.7
update-alternatives: removing manually selected alternative - switching python to auto mode
update-alternatives: using /usr/bin/python3.8 to provide /usr/bin/python (python) in auto mode

По мотивам этой статьи.

Ответы:

sudo update-alternatives --config python

update-alternatives: error: no alternatives for python3 

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2

sudo update-alternatives --config python

sudo update-alternatives  --set python /usr/bin/python3.6

# ls -lh /usr/bin/python3 /usr/bin/python
lrwxrwxrwx 1 root root 9 ноя 24  2017 /usr/bin/python -> python2.7
lrwxrwxrwx 1 root root 9 сен  6  2017 /usr/bin/python3 -> python3.5

# mv /usr/bin/python
# cp /usr/bin/python3 /usr/bin/python
# python --version
Python 3.5.2

root@irshad:/usr/bin# ls -lrth python*
lrwxrwxrwx 1 root root    9 Apr 16  2018 python -> python2.7
-rwxr-xr-x 1 root root 3.6M Nov 12  2018 python2.7
-rwxr-xr-x 2 root root 4.4M May  7 14:58 python3.6

root@irshad:/usr/bin# unlink python
root@irshad:/usr/bin# ln -s /usr/bin/python3.6 python
root@irshad:/usr/bin# python --version
Python 3.6.8

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 2

update-alternatives --config python3

sudo update-alternatives --install <symlink_origin> <name_of_config> <symlink_destination> <priority>